Grainger. . I did a throttling valve for mine and for around $50. 00 you can get a manual valve that is spring loaded to off (or on, however you plumb it) It took some creativity to make a mount for it but it works like a champ. . I have 5 Kleinn's, a 2 chime and a 3 chime. The 3 chime is plumbed with 3/8" air brake tubing and comression fittings, it is fed from the manual valve. The 2 chime has the 2 solenoids it came with and is plumbed with 1/4" air brake tubing from the install kit. The 3 chime also has a single solenoid and is hooked to the same source as the 2 chime set with 1/4" tubing. when the horn button is depressed all 5 horns activate (Pretty much with out delay) and dang are they loud. the manual valve allows me to feather in the 3 chime set. If the horn button is depressed and the manual valve is feathered in at the same time the output is louder and changes pitch. This would be because all 5 are now fed with the 3'8" tubing as well. I would not waste my money on the "Louder" installation kit with 1/2" tubing, on the Kleinn's there is only a 1/8" NPT port on the horns anyway, using 3/8" is about all you can get out of them. They do rattle the windows and best of all you cannot see where they are, it is pretty good for the 'shock and awe' value. At 159 db at 3 meters it works.